The present invention relates to a closed loop mixture control system for carburetor-equipped internal combustion engines.
The three-way catalyst allows simultaneous conversion of hydrocarbons, carbon monoxide and oxides of nitrogen into harmless wastes. For the catalyst to perform satisfactorily, the engine must have a very closely controlled air-fuel ratio. In fact, a slight offset from the stoichiometric air-fuel ratio can result in a greater reduction in conversion efficiency. A standard carburetor cannot adapt adequately to the variations in engine operation and will thus not give the catalyst the quality of input needed.
To provide precise air-fuel ratio control, a feedback control scheme has been designed for the carburetor using an exhaust gas sensor placed in the exhaust manifold. Whenever the engine is running outside the ideal air-fuel ratio, the gas sensor sends signals to an electronic control unit, which then corrects the fuel-metering system, or carburetor.
However, since the air-fuel ratio varies widely between carburetors because of their different operating characteristics, precise calibration would be required to fit the feedback control loop to the characteristics of each carburetor. Furthermore, since the operating characteristics of the carburetor also vary as a function of aging, such calibration would thus be required at periodic intervals to check if the feedback control is operating satisfactorily.